During hypo- and hyperbaric decompression, gas-supersaturation of tissues and blood can cause bubbles to form there with serious medical consequences. The growing use of hyperbaric environments by man has made this problem of air embolism increasingly important. The objective of the present physiological study is to increase the understanding of the early etiology of air embolism, an area which has received relatively little attention in the past. Essential information is lacking as to where and how the bubbles form, and the factors which cause the dramatic lowering of liquid cavitation stability in vivo are still to be established. Several fundamental aspects of gas bubble nucleation and initial growth will be investigated by cavitation experiments on solutions, interface systems, biological fluids, microorganisms, and tissue preparations. The basic methods and procedures which have been developed in the present phase of these studies will be supplemented by high-speed cinemicrographic recordings of the cavitation events. This will allow detailed determinations of the loci for the appearance of bubbles, and their microscopic rate of growth for various pressures and decompression rates. It is of further interest to determine both the possible lower supersaturation ranges required for true nucleation in various in vivo systems and the physical parameters which govern this process. The latter will be of particular value for reaching a quantitative, theoretical formulation of the nucleation events. Our long range goal is to aid in the prevention and treatment of air embolism by contributing to the development of better hyperbaric gas mixtures and decompression procedures.